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Quantification of Flow Dynamics in Congenital Heart Disease: Applications of Velocity-encoded Cine MR Imaging¹

¹ From the Department of Radiology, Box 0628, University of California, San Francisco, 505 Parnassus Ave, Suite L325, San Francisco, CA 94143-0628. Received October 19, 2001; revision requested January 11, 2002 and received February 20; accepted February 22. Address correspondence to G.P.R. (e-mail: gautham.reddy@radiology.ucsf.edu).

	Abstract

Top Abstract Introduction Imaging Techniques Imaging Limitations and Pitfalls Clinical Applications Conclusions References

 
Velocity-encoded cine (VEC) magnetic resonance (MR) imaging is a valuable technique for quantitative assessment of flow dynamics in congenital heart disease (CHD). VEC MR imaging has a variety of clinical applications, including the measurement of collateral flow and pressure gradients in coarctation of the aorta, differentiation of blood flow in the left and right pulmonary arteries, quantification of shunts, and evaluation of valvular regurgitation and stenosis. After surgical repair of CHD, VEC MR imaging can be used to monitor conduit blood flow, stenosis, and flow dynamics. There are some pitfalls that can occur in VEC MR imaging. These include potential underestimation of velocity and flow, aliasing, inadequate depiction of very small vessels, and possible errors in pressure gradient measurements. Nevertheless, VEC MR imaging is a valuable tool for preoperative planning and postoperative monitoring in patients with CHD.

© RSNA, 2002

Index Terms: Aorta, flow dynamics • Aorta, MR, 562.12144 • Aorta, stenosis or obstruction, 562.1511 • Heart, diseases, 51.1424, 51.191, 53.172 Heart, flow dynamics • Heart, MR, 51.12144 • Magnetic resonance (MR), cine study, 50.12144 • Magnetic resonance (MR), vascular studies, 564.12144 • Pulmonary arteries, flow dynamics • Pulmonary arteries, MR, 564.12144

	Introduction

Top Abstract Introduction Imaging Techniques Imaging Limitations and Pitfalls Clinical Applications Conclusions References

 
Magnetic resonance (MR) imaging was initially used to demonstrate the anatomy of patients with congenital heart disease (CHD) (1–4). In recent years, it has become apparent that velocity-encoded cine (VEC) MR imaging is useful for quantitative assessment of the heart and great vessels (5–7). In this article, we discuss the techniques as well as the limitations of VEC MR imaging. We also discuss and illustrate some of its clinical applications, including functional evaluation of coarctation of the aorta, assessment of differential pulmonary arterial flow, shunt quantification, and evaluation of valvular disease.

	Imaging Techniques

Top Abstract Introduction Imaging Techniques Imaging Limitations and Pitfalls Clinical Applications Conclusions References

 
The morphologic aspects of CHD are usually evaluated initially with electrocardiographically (EKG)–gated spin-echo tomographic imaging and gadolinium-enhanced three-dimensional MR angiography.

In patients with CHD, evaluation of function is at least as important as delineation of anatomy. Cine MR imaging can be performed to evaluate left and right ventricular volume, mass, and function (8). Flow-sensitive MR imaging can be performed for quantitative assessment of cardiovascular physiologic characteristics and flow dynamics.

Currently, the most frequently used flow-sensitive MR imaging technique is VEC MR imaging (Fig 1). This is a phase-contrast sequence based on the principle that the phase of flowing spins relative to stationary spins along a magnetic gradient changes in proportion to flow velocity; in other words, protons in motion change phase angle in proportion to velocity (9). Change in phase angle is mapped on phase images, and flow velocity is computed with a standard formula. The vessel of interest is assessed in a plane perpendicular to the flow vector (10). To determine flow volume across a vessel lumen, the spatial mean velocity of the vessel of interest is multiplied by the cross-sectional area of the vessel.

View larger version (110K): [in this window] [in a new window] [Download PPT slide]   Figure 1a.  VEC MR imaging flow measurements. VEC MR imaging is used to assess the vessel of interest in a plane perpendicular to the flow vector. (a, b) Magnitude (a) and phase (b) VEC MR images are obtained in a plane perpendicular to the direction of flow in the ascending aorta (A). D = descending aorta. (c) Diagram illustrates a velocity-time curve for blood flow within the aorta. The five circles represent the appearance of the blood vessel at selected phases during the cardiac cycle (points on curve). The shade of each circle indicates the amount of blood flow in the artery at that point of the cycle (black = highest flow, pale gray = lowest flow). Each point represents the mean blood flow within the artery at a given phase, and each value is derived from a separate VEC MR image. To determine flow volume through the vessel lumen, the spatial mean velocity in the vessel of interest is multiplied by the cross-sectional area of the vessel. (d) Magnitude (top row) and phase (bottom row) VEC MR images obtained at four of 16 points during the cardiac cycle demonstrate how a region of interest is drawn around the ascending aorta on each magnitude image; each region of interest should be transferred to the corresponding phase image. Flow is then calculated for each phase image (flow volume = spatial mean velocity x cross-sectional area of the vessel). Volumes for all 16 time points are summed to obtain total flow volume.

View larger version (114K): [in this window] [in a new window] [Download PPT slide]   Figure 1b.  VEC MR imaging flow measurements. VEC MR imaging is used to assess the vessel of interest in a plane perpendicular to the flow vector. (a, b) Magnitude (a) and phase (b) VEC MR images are obtained in a plane perpendicular to the direction of flow in the ascending aorta (A). D = descending aorta. (c) Diagram illustrates a velocity-time curve for blood flow within the aorta. The five circles represent the appearance of the blood vessel at selected phases during the cardiac cycle (points on curve). The shade of each circle indicates the amount of blood flow in the artery at that point of the cycle (black = highest flow, pale gray = lowest flow). Each point represents the mean blood flow within the artery at a given phase, and each value is derived from a separate VEC MR image. To determine flow volume through the vessel lumen, the spatial mean velocity in the vessel of interest is multiplied by the cross-sectional area of the vessel. (d) Magnitude (top row) and phase (bottom row) VEC MR images obtained at four of 16 points during the cardiac cycle demonstrate how a region of interest is drawn around the ascending aorta on each magnitude image; each region of interest should be transferred to the corresponding phase image. Flow is then calculated for each phase image (flow volume = spatial mean velocity x cross-sectional area of the vessel). Volumes for all 16 time points are summed to obtain total flow volume.

View larger version (31K): [in this window] [in a new window] [Download PPT slide]   Figure 1c.  VEC MR imaging flow measurements. VEC MR imaging is used to assess the vessel of interest in a plane perpendicular to the flow vector. (a, b) Magnitude (a) and phase (b) VEC MR images are obtained in a plane perpendicular to the direction of flow in the ascending aorta (A). D = descending aorta. (c) Diagram illustrates a velocity-time curve for blood flow within the aorta. The five circles represent the appearance of the blood vessel at selected phases during the cardiac cycle (points on curve). The shade of each circle indicates the amount of blood flow in the artery at that point of the cycle (black = highest flow, pale gray = lowest flow). Each point represents the mean blood flow within the artery at a given phase, and each value is derived from a separate VEC MR image. To determine flow volume through the vessel lumen, the spatial mean velocity in the vessel of interest is multiplied by the cross-sectional area of the vessel. (d) Magnitude (top row) and phase (bottom row) VEC MR images obtained at four of 16 points during the cardiac cycle demonstrate how a region of interest is drawn around the ascending aorta on each magnitude image; each region of interest should be transferred to the corresponding phase image. Flow is then calculated for each phase image (flow volume = spatial mean velocity x cross-sectional area of the vessel). Volumes for all 16 time points are summed to obtain total flow volume.

View larger version (180K): [in this window] [in a new window] [Download PPT slide]   Figure 1d.  VEC MR imaging flow measurements. VEC MR imaging is used to assess the vessel of interest in a plane perpendicular to the flow vector. (a, b) Magnitude (a) and phase (b) VEC MR images are obtained in a plane perpendicular to the direction of flow in the ascending aorta (A). D = descending aorta. (c) Diagram illustrates a velocity-time curve for blood flow within the aorta. The five circles represent the appearance of the blood vessel at selected phases during the cardiac cycle (points on curve). The shade of each circle indicates the amount of blood flow in the artery at that point of the cycle (black = highest flow, pale gray = lowest flow). Each point represents the mean blood flow within the artery at a given phase, and each value is derived from a separate VEC MR image. To determine flow volume through the vessel lumen, the spatial mean velocity in the vessel of interest is multiplied by the cross-sectional area of the vessel. (d) Magnitude (top row) and phase (bottom row) VEC MR images obtained at four of 16 points during the cardiac cycle demonstrate how a region of interest is drawn around the ascending aorta on each magnitude image; each region of interest should be transferred to the corresponding phase image. Flow is then calculated for each phase image (flow volume = spatial mean velocity x cross-sectional area of the vessel). Volumes for all 16 time points are summed to obtain total flow volume.

	Imaging Limitations and Pitfalls

Top Abstract Introduction Imaging Techniques Imaging Limitations and Pitfalls Clinical Applications Conclusions References

 
There are some pitfalls inherent in VEC MR imaging. Underestimation of velocity and flow can result if the vessel of interest is not imaged in a plane perpendicular to flow or if partial volume averaging occurs (7,11). Aliasing may occur if the selected maximum velocity is lower than the actual peak velocity at any time during the cardiac cycle (7). In addition, very small vessels are poorly evaluated with VEC MR imaging because they occupy few pixels. If the vessel diameter is less than 2–2.5 mm (four to eight pixels), accuracy of flow measurements may suffer (12).

Pressure gradient measurements are also subject to error. In cases of vessel stenosis, the site of peak velocity may be distal to the most severe stenosis; if the vessel is not interrogated at this site, the peak pressure gradient can be underestimated (13). The limited temporal resolution of VEC MR imaging can also lead to underestimation of peak velocity and therefore of the pressure gradient.

	Clinical Applications

Top Abstract Introduction Imaging Techniques Imaging Limitations and Pitfalls Clinical Applications Conclusions References

 
Evaluation of Coarctation of the Aorta
The physiologic severity of coarctation of the aorta can be measured with VEC MR imaging using two different methods (14,15). In the first method, VEC MR imaging is used to determine the quantity of collateral blood flow. In healthy individuals, flow volume in the proximal descending aorta is slightly (approximately 7%) higher than in the distal thoracic aorta (15), but in patients with coarctation, there may be greater blood flow distally due to retrograde collateral blood flow into the aorta from the intercostal arteries and other aortic branches. Flow volume is estimated at two locations in the aorta: one just distal to the coarctation site, and the other at the level of the diaphragm (15). Collateral flow is present if distal aortic flow is greater than proximal flow. To determine the quantity of collateral circulation, flow volume in the proximal descending aorta is subtracted from that in the distal aorta (Fig 2). The presence of collateral flow indicates a hemodynamically significant coarctation.

View larger version (144K): [in this window] [in a new window] [Download PPT slide]   Figure 2a.  Coarctation of the aorta. (a, b) Estimation of collateral circulation and calculation of the pressure gradient across a coarctation. Oblique sagittal EKG-gated spin-echo MR images obtained in an 11-year-old girl show a severe, discrete juxtaductal coarctation (arrow in a) and the planes orthogonal to the aorta in which VEC MR imaging is performed (b). Imaging planes for measurement of collateral flow intersect the proximal descending aorta just distal to the coarctation (P) and the distal descending aorta (D). VEC MR imaging is performed orthogonal to the aorta at the site of the coarctation (C) to calculate the pressure gradient. (c-h) Measurement of collateral flow. (c-f) Magnitude (c, e) and phase (d, f) VEC MR images are acquired in the proximal (c, d) and distal (e, f) aorta (arrow). Regions of interest are then drawn around the aorta on the magnitude images, and flow in the proximal and distal aorta is calculated with the phase images. (g) Graph illustrates aortic flow in a patient without coarctation of the aorta. The flow curve is calculated by plotting blood flow in the proximal and distal aorta against time. In healthy individuals, flow in the proximal aorta is slightly greater than flow in the distal aorta. (h) Graph illustrates aortic flow in a patient with severe coarctation of the aorta. Patients with hemodynamically significant coarctation have greater flow distally than proximally due to collateral flow, which is quantified by subtracting proximal flow volume from distal flow volume. The patient in this case had twice as much blood flow in the distal aorta as in the proximal descending aorta, indicating that the volume of collateral circulation was approximately equal to flow just beyond the coarctation. (i) Calculation of the pressure gradient across the coarctation. Graph illustrates how peak velocity across the coarctation is plotted against time. The pressure gradient across the coarctation is calculated with the modified Bernoulli equation P = 4v2, where P is the pressure gradient and v is the peak flow velocity. In this case, the peak velocity was 3.2 m/sec and the pressure gradient was 41 mm Hg, which is considered severe. The patient underwent surgical repair of the coarctation.

View larger version (146K): [in this window] [in a new window] [Download PPT slide]   Figure 2b.  Coarctation of the aorta. (a, b) Estimation of collateral circulation and calculation of the pressure gradient across a coarctation. Oblique sagittal EKG-gated spin-echo MR images obtained in an 11-year-old girl show a severe, discrete juxtaductal coarctation (arrow in a) and the planes orthogonal to the aorta in which VEC MR imaging is performed (b). Imaging planes for measurement of collateral flow intersect the proximal descending aorta just distal to the coarctation (P) and the distal descending aorta (D). VEC MR imaging is performed orthogonal to the aorta at the site of the coarctation (C) to calculate the pressure gradient. (c-h) Measurement of collateral flow. (c-f) Magnitude (c, e) and phase (d, f) VEC MR images are acquired in the proximal (c, d) and distal (e, f) aorta (arrow). Regions of interest are then drawn around the aorta on the magnitude images, and flow in the proximal and distal aorta is calculated with the phase images. (g) Graph illustrates aortic flow in a patient without coarctation of the aorta. The flow curve is calculated by plotting blood flow in the proximal and distal aorta against time. In healthy individuals, flow in the proximal aorta is slightly greater than flow in the distal aorta. (h) Graph illustrates aortic flow in a patient with severe coarctation of the aorta. Patients with hemodynamically significant coarctation have greater flow distally than proximally due to collateral flow, which is quantified by subtracting proximal flow volume from distal flow volume. The patient in this case had twice as much blood flow in the distal aorta as in the proximal descending aorta, indicating that the volume of collateral circulation was approximately equal to flow just beyond the coarctation. (i) Calculation of the pressure gradient across the coarctation. Graph illustrates how peak velocity across the coarctation is plotted against time. The pressure gradient across the coarctation is calculated with the modified Bernoulli equation P = 4v2, where P is the pressure gradient and v is the peak flow velocity. In this case, the peak velocity was 3.2 m/sec and the pressure gradient was 41 mm Hg, which is considered severe. The patient underwent surgical repair of the coarctation.

View larger version (128K): [in this window] [in a new window] [Download PPT slide]   Figure 2c.  Coarctation of the aorta. (a, b) Estimation of collateral circulation and calculation of the pressure gradient across a coarctation. Oblique sagittal EKG-gated spin-echo MR images obtained in an 11-year-old girl show a severe, discrete juxtaductal coarctation (arrow in a) and the planes orthogonal to the aorta in which VEC MR imaging is performed (b). Imaging planes for measurement of collateral flow intersect the proximal descending aorta just distal to the coarctation (P) and the distal descending aorta (D). VEC MR imaging is performed orthogonal to the aorta at the site of the coarctation (C) to calculate the pressure gradient. (c-h) Measurement of collateral flow. (c-f) Magnitude (c, e) and phase (d, f) VEC MR images are acquired in the proximal (c, d) and distal (e, f) aorta (arrow). Regions of interest are then drawn around the aorta on the magnitude images, and flow in the proximal and distal aorta is calculated with the phase images. (g) Graph illustrates aortic flow in a patient without coarctation of the aorta. The flow curve is calculated by plotting blood flow in the proximal and distal aorta against time. In healthy individuals, flow in the proximal aorta is slightly greater than flow in the distal aorta. (h) Graph illustrates aortic flow in a patient with severe coarctation of the aorta. Patients with hemodynamically significant coarctation have greater flow distally than proximally due to collateral flow, which is quantified by subtracting proximal flow volume from distal flow volume. The patient in this case had twice as much blood flow in the distal aorta as in the proximal descending aorta, indicating that the volume of collateral circulation was approximately equal to flow just beyond the coarctation. (i) Calculation of the pressure gradient across the coarctation. Graph illustrates how peak velocity across the coarctation is plotted against time. The pressure gradient across the coarctation is calculated with the modified Bernoulli equation P = 4v2, where P is the pressure gradient and v is the peak flow velocity. In this case, the peak velocity was 3.2 m/sec and the pressure gradient was 41 mm Hg, which is considered severe. The patient underwent surgical repair of the coarctation.

View larger version (112K): [in this window] [in a new window] [Download PPT slide]   Figure 2d.  Coarctation of the aorta. (a, b) Estimation of collateral circulation and calculation of the pressure gradient across a coarctation. Oblique sagittal EKG-gated spin-echo MR images obtained in an 11-year-old girl show a severe, discrete juxtaductal coarctation (arrow in a) and the planes orthogonal to the aorta in which VEC MR imaging is performed (b). Imaging planes for measurement of collateral flow intersect the proximal descending aorta just distal to the coarctation (P) and the distal descending aorta (D). VEC MR imaging is performed orthogonal to the aorta at the site of the coarctation (C) to calculate the pressure gradient. (c-h) Measurement of collateral flow. (c-f) Magnitude (c, e) and phase (d, f) VEC MR images are acquired in the proximal (c, d) and distal (e, f) aorta (arrow). Regions of interest are then drawn around the aorta on the magnitude images, and flow in the proximal and distal aorta is calculated with the phase images. (g) Graph illustrates aortic flow in a patient without coarctation of the aorta. The flow curve is calculated by plotting blood flow in the proximal and distal aorta against time. In healthy individuals, flow in the proximal aorta is slightly greater than flow in the distal aorta. (h) Graph illustrates aortic flow in a patient with severe coarctation of the aorta. Patients with hemodynamically significant coarctation have greater flow distally than proximally due to collateral flow, which is quantified by subtracting proximal flow volume from distal flow volume. The patient in this case had twice as much blood flow in the distal aorta as in the proximal descending aorta, indicating that the volume of collateral circulation was approximately equal to flow just beyond the coarctation. (i) Calculation of the pressure gradient across the coarctation. Graph illustrates how peak velocity across the coarctation is plotted against time. The pressure gradient across the coarctation is calculated with the modified Bernoulli equation P = 4v2, where P is the pressure gradient and v is the peak flow velocity. In this case, the peak velocity was 3.2 m/sec and the pressure gradient was 41 mm Hg, which is considered severe. The patient underwent surgical repair of the coarctation.

View larger version (141K): [in this window] [in a new window] [Download PPT slide]   Figure 2e.  Coarctation of the aorta. (a, b) Estimation of collateral circulation and calculation of the pressure gradient across a coarctation. Oblique sagittal EKG-gated spin-echo MR images obtained in an 11-year-old girl show a severe, discrete juxtaductal coarctation (arrow in a) and the planes orthogonal to the aorta in which VEC MR imaging is performed (b). Imaging planes for measurement of collateral flow intersect the proximal descending aorta just distal to the coarctation (P) and the distal descending aorta (D). VEC MR imaging is performed orthogonal to the aorta at the site of the coarctation (C) to calculate the pressure gradient. (c-h) Measurement of collateral flow. (c-f) Magnitude (c, e) and phase (d, f) VEC MR images are acquired in the proximal (c, d) and distal (e, f) aorta (arrow). Regions of interest are then drawn around the aorta on the magnitude images, and flow in the proximal and distal aorta is calculated with the phase images. (g) Graph illustrates aortic flow in a patient without coarctation of the aorta. The flow curve is calculated by plotting blood flow in the proximal and distal aorta against time. In healthy individuals, flow in the proximal aorta is slightly greater than flow in the distal aorta. (h) Graph illustrates aortic flow in a patient with severe coarctation of the aorta. Patients with hemodynamically significant coarctation have greater flow distally than proximally due to collateral flow, which is quantified by subtracting proximal flow volume from distal flow volume. The patient in this case had twice as much blood flow in the distal aorta as in the proximal descending aorta, indicating that the volume of collateral circulation was approximately equal to flow just beyond the coarctation. (i) Calculation of the pressure gradient across the coarctation. Graph illustrates how peak velocity across the coarctation is plotted against time. The pressure gradient across the coarctation is calculated with the modified Bernoulli equation P = 4v2, where P is the pressure gradient and v is the peak flow velocity. In this case, the peak velocity was 3.2 m/sec and the pressure gradient was 41 mm Hg, which is considered severe. The patient underwent surgical repair of the coarctation.

View larger version (106K): [in this window] [in a new window] [Download PPT slide]   Figure 2f.  Coarctation of the aorta. (a, b) Estimation of collateral circulation and calculation of the pressure gradient across a coarctation. Oblique sagittal EKG-gated spin-echo MR images obtained in an 11-year-old girl show a severe, discrete juxtaductal coarctation (arrow in a) and the planes orthogonal to the aorta in which VEC MR imaging is performed (b). Imaging planes for measurement of collateral flow intersect the proximal descending aorta just distal to the coarctation (P) and the distal descending aorta (D). VEC MR imaging is performed orthogonal to the aorta at the site of the coarctation (C) to calculate the pressure gradient. (c-h) Measurement of collateral flow. (c-f) Magnitude (c, e) and phase (d, f) VEC MR images are acquired in the proximal (c, d) and distal (e, f) aorta (arrow). Regions of interest are then drawn around the aorta on the magnitude images, and flow in the proximal and distal aorta is calculated with the phase images. (g) Graph illustrates aortic flow in a patient without coarctation of the aorta. The flow curve is calculated by plotting blood flow in the proximal and distal aorta against time. In healthy individuals, flow in the proximal aorta is slightly greater than flow in the distal aorta. (h) Graph illustrates aortic flow in a patient with severe coarctation of the aorta. Patients with hemodynamically significant coarctation have greater flow distally than proximally due to collateral flow, which is quantified by subtracting proximal flow volume from distal flow volume. The patient in this case had twice as much blood flow in the distal aorta as in the proximal descending aorta, indicating that the volume of collateral circulation was approximately equal to flow just beyond the coarctation. (i) Calculation of the pressure gradient across the coarctation. Graph illustrates how peak velocity across the coarctation is plotted against time. The pressure gradient across the coarctation is calculated with the modified Bernoulli equation P = 4v2, where P is the pressure gradient and v is the peak flow velocity. In this case, the peak velocity was 3.2 m/sec and the pressure gradient was 41 mm Hg, which is considered severe. The patient underwent surgical repair of the coarctation.

View larger version (30K): [in this window] [in a new window] [Download PPT slide]   Figure 2g.  Coarctation of the aorta. (a, b) Estimation of collateral circulation and calculation of the pressure gradient across a coarctation. Oblique sagittal EKG-gated spin-echo MR images obtained in an 11-year-old girl show a severe, discrete juxtaductal coarctation (arrow in a) and the planes orthogonal to the aorta in which VEC MR imaging is performed (b). Imaging planes for measurement of collateral flow intersect the proximal descending aorta just distal to the coarctation (P) and the distal descending aorta (D). VEC MR imaging is performed orthogonal to the aorta at the site of the coarctation (C) to calculate the pressure gradient. (c-h) Measurement of collateral flow. (c-f) Magnitude (c, e) and phase (d, f) VEC MR images are acquired in the proximal (c, d) and distal (e, f) aorta (arrow). Regions of interest are then drawn around the aorta on the magnitude images, and flow in the proximal and distal aorta is calculated with the phase images. (g) Graph illustrates aortic flow in a patient without coarctation of the aorta. The flow curve is calculated by plotting blood flow in the proximal and distal aorta against time. In healthy individuals, flow in the proximal aorta is slightly greater than flow in the distal aorta. (h) Graph illustrates aortic flow in a patient with severe coarctation of the aorta. Patients with hemodynamically significant coarctation have greater flow distally than proximally due to collateral flow, which is quantified by subtracting proximal flow volume from distal flow volume. The patient in this case had twice as much blood flow in the distal aorta as in the proximal descending aorta, indicating that the volume of collateral circulation was approximately equal to flow just beyond the coarctation. (i) Calculation of the pressure gradient across the coarctation. Graph illustrates how peak velocity across the coarctation is plotted against time. The pressure gradient across the coarctation is calculated with the modified Bernoulli equation P = 4v2, where P is the pressure gradient and v is the peak flow velocity. In this case, the peak velocity was 3.2 m/sec and the pressure gradient was 41 mm Hg, which is considered severe. The patient underwent surgical repair of the coarctation.

View larger version (27K): [in this window] [in a new window] [Download PPT slide]   Figure 2h.  Coarctation of the aorta. (a, b) Estimation of collateral circulation and calculation of the pressure gradient across a coarctation. Oblique sagittal EKG-gated spin-echo MR images obtained in an 11-year-old girl show a severe, discrete juxtaductal coarctation (arrow in a) and the planes orthogonal to the aorta in which VEC MR imaging is performed (b). Imaging planes for measurement of collateral flow intersect the proximal descending aorta just distal to the coarctation (P) and the distal descending aorta (D). VEC MR imaging is performed orthogonal to the aorta at the site of the coarctation (C) to calculate the pressure gradient. (c-h) Measurement of collateral flow. (c-f) Magnitude (c, e) and phase (d, f) VEC MR images are acquired in the proximal (c, d) and distal (e, f) aorta (arrow). Regions of interest are then drawn around the aorta on the magnitude images, and flow in the proximal and distal aorta is calculated with the phase images. (g) Graph illustrates aortic flow in a patient without coarctation of the aorta. The flow curve is calculated by plotting blood flow in the proximal and distal aorta against time. In healthy individuals, flow in the proximal aorta is slightly greater than flow in the distal aorta. (h) Graph illustrates aortic flow in a patient with severe coarctation of the aorta. Patients with hemodynamically significant coarctation have greater flow distally than proximally due to collateral flow, which is quantified by subtracting proximal flow volume from distal flow volume. The patient in this case had twice as much blood flow in the distal aorta as in the proximal descending aorta, indicating that the volume of collateral circulation was approximately equal to flow just beyond the coarctation. (i) Calculation of the pressure gradient across the coarctation. Graph illustrates how peak velocity across the coarctation is plotted against time. The pressure gradient across the coarctation is calculated with the modified Bernoulli equation P = 4v2, where P is the pressure gradient and v is the peak flow velocity. In this case, the peak velocity was 3.2 m/sec and the pressure gradient was 41 mm Hg, which is considered severe. The patient underwent surgical repair of the coarctation.

View larger version (21K): [in this window] [in a new window] [Download PPT slide]   Figure 2i.  Coarctation of the aorta. (a, b) Estimation of collateral circulation and calculation of the pressure gradient across a coarctation. Oblique sagittal EKG-gated spin-echo MR images obtained in an 11-year-old girl show a severe, discrete juxtaductal coarctation (arrow in a) and the planes orthogonal to the aorta in which VEC MR imaging is performed (b). Imaging planes for measurement of collateral flow intersect the proximal descending aorta just distal to the coarctation (P) and the distal descending aorta (D). VEC MR imaging is performed orthogonal to the aorta at the site of the coarctation (C) to calculate the pressure gradient. (c-h) Measurement of collateral flow. (c-f) Magnitude (c, e) and phase (d, f) VEC MR images are acquired in the proximal (c, d) and distal (e, f) aorta (arrow). Regions of interest are then drawn around the aorta on the magnitude images, and flow in the proximal and distal aorta is calculated with the phase images. (g) Graph illustrates aortic flow in a patient without coarctation of the aorta. The flow curve is calculated by plotting blood flow in the proximal and distal aorta against time. In healthy individuals, flow in the proximal aorta is slightly greater than flow in the distal aorta. (h) Graph illustrates aortic flow in a patient with severe coarctation of the aorta. Patients with hemodynamically significant coarctation have greater flow distally than proximally due to collateral flow, which is quantified by subtracting proximal flow volume from distal flow volume. The patient in this case had twice as much blood flow in the distal aorta as in the proximal descending aorta, indicating that the volume of collateral circulation was approximately equal to flow just beyond the coarctation. (i) Calculation of the pressure gradient across the coarctation. Graph illustrates how peak velocity across the coarctation is plotted against time. The pressure gradient across the coarctation is calculated with the modified Bernoulli equation P = 4v2, where P is the pressure gradient and v is the peak flow velocity. In this case, the peak velocity was 3.2 m/sec and the pressure gradient was 41 mm Hg, which is considered severe. The patient underwent surgical repair of the coarctation.

The pressure gradient calculated with VEC MR imaging can be used to determine the need for repair of coarctation of the aorta. Typically, a pressure gradient greater than 15 mm Hg is considered an indication for intervention. However, this threshold is arbitrary. Therefore, it may be more appropriate to use the measurement of collateral flow.

Assessment of Differential Pulmonary Arterial Flow
VEC MR imaging has the unique capacity to measure blood flow separately in the left and right pulmonary arteries (16). Differential pulmonary blood flow can be quantified in cases of unequal pulmonary flow, such as in patients with stenotic or hypoplastic pulmonary arteries (Fig 3), or after connection of surgical conduits to the central pulmonary arteries. This technique can also be used in patients with pulmonary artery sling to assess the distribution of flow to the lungs (Fig 4) (17).

View larger version (130K): [in this window] [in a new window] [Download PPT slide]   Figure 3a.  Differential right and left pulmonary arterial blood flow measurements in a woman with left lung hypoplasia. (a, b) Axial breath-hold gradient-echo cine MR images demonstrate an enlarged right pulmonary artery (R) and a hypoplastic left pulmonary artery (L). To calculate pulmonary arterial flow volumes, VEC MR imaging is performed in planes orthogonal to the right and left pulmonary arteries (black line). (c-g) Blood flow measurements. Magnitude (c, e) and phase (d, f) VEC MR images are obtained through the right (c, d) and left (e, f) pulmonary arteries (arrow). Regions of interest are then drawn around the pulmonary arteries, and flow in these vessels is calculated with the phase images. Ao = aorta. (g) Graph illustrates how arterial flow is plotted against time. The area under each curve represents the flow volume for a cardiac cycle. In this case, the right and left pulmonary arteries (PA) received 78% and 22% of the pulmonary flow, respectively.

View larger version (118K): [in this window] [in a new window] [Download PPT slide]   Figure 3b.  Differential right and left pulmonary arterial blood flow measurements in a woman with left lung hypoplasia. (a, b) Axial breath-hold gradient-echo cine MR images demonstrate an enlarged right pulmonary artery (R) and a hypoplastic left pulmonary artery (L). To calculate pulmonary arterial flow volumes, VEC MR imaging is performed in planes orthogonal to the right and left pulmonary arteries (black line). (c-g) Blood flow measurements. Magnitude (c, e) and phase (d, f) VEC MR images are obtained through the right (c, d) and left (e, f) pulmonary arteries (arrow). Regions of interest are then drawn around the pulmonary arteries, and flow in these vessels is calculated with the phase images. Ao = aorta. (g) Graph illustrates how arterial flow is plotted against time. The area under each curve represents the flow volume for a cardiac cycle. In this case, the right and left pulmonary arteries (PA) received 78% and 22% of the pulmonary flow, respectively.

View larger version (157K): [in this window] [in a new window] [Download PPT slide]   Figure 3c.  Differential right and left pulmonary arterial blood flow measurements in a woman with left lung hypoplasia. (a, b) Axial breath-hold gradient-echo cine MR images demonstrate an enlarged right pulmonary artery (R) and a hypoplastic left pulmonary artery (L). To calculate pulmonary arterial flow volumes, VEC MR imaging is performed in planes orthogonal to the right and left pulmonary arteries (black line). (c-g) Blood flow measurements. Magnitude (c, e) and phase (d, f) VEC MR images are obtained through the right (c, d) and left (e, f) pulmonary arteries (arrow). Regions of interest are then drawn around the pulmonary arteries, and flow in these vessels is calculated with the phase images. Ao = aorta. (g) Graph illustrates how arterial flow is plotted against time. The area under each curve represents the flow volume for a cardiac cycle. In this case, the right and left pulmonary arteries (PA) received 78% and 22% of the pulmonary flow, respectively.

View larger version (154K): [in this window] [in a new window] [Download PPT slide]   Figure 3d.  Differential right and left pulmonary arterial blood flow measurements in a woman with left lung hypoplasia. (a, b) Axial breath-hold gradient-echo cine MR images demonstrate an enlarged right pulmonary artery (R) and a hypoplastic left pulmonary artery (L). To calculate pulmonary arterial flow volumes, VEC MR imaging is performed in planes orthogonal to the right and left pulmonary arteries (black line). (c-g) Blood flow measurements. Magnitude (c, e) and phase (d, f) VEC MR images are obtained through the right (c, d) and left (e, f) pulmonary arteries (arrow). Regions of interest are then drawn around the pulmonary arteries, and flow in these vessels is calculated with the phase images. Ao = aorta. (g) Graph illustrates how arterial flow is plotted against time. The area under each curve represents the flow volume for a cardiac cycle. In this case, the right and left pulmonary arteries (PA) received 78% and 22% of the pulmonary flow, respectively.

View larger version (137K): [in this window] [in a new window] [Download PPT slide]   Figure 3e.  Differential right and left pulmonary arterial blood flow measurements in a woman with left lung hypoplasia. (a, b) Axial breath-hold gradient-echo cine MR images demonstrate an enlarged right pulmonary artery (R) and a hypoplastic left pulmonary artery (L). To calculate pulmonary arterial flow volumes, VEC MR imaging is performed in planes orthogonal to the right and left pulmonary arteries (black line). (c-g) Blood flow measurements. Magnitude (c, e) and phase (d, f) VEC MR images are obtained through the right (c, d) and left (e, f) pulmonary arteries (arrow). Regions of interest are then drawn around the pulmonary arteries, and flow in these vessels is calculated with the phase images. Ao = aorta. (g) Graph illustrates how arterial flow is plotted against time. The area under each curve represents the flow volume for a cardiac cycle. In this case, the right and left pulmonary arteries (PA) received 78% and 22% of the pulmonary flow, respectively.

View larger version (99K): [in this window] [in a new window] [Download PPT slide]   Figure 3f.  Differential right and left pulmonary arterial blood flow measurements in a woman with left lung hypoplasia. (a, b) Axial breath-hold gradient-echo cine MR images demonstrate an enlarged right pulmonary artery (R) and a hypoplastic left pulmonary artery (L). To calculate pulmonary arterial flow volumes, VEC MR imaging is performed in planes orthogonal to the right and left pulmonary arteries (black line). (c-g) Blood flow measurements. Magnitude (c, e) and phase (d, f) VEC MR images are obtained through the right (c, d) and left (e, f) pulmonary arteries (arrow). Regions of interest are then drawn around the pulmonary arteries, and flow in these vessels is calculated with the phase images. Ao = aorta. (g) Graph illustrates how arterial flow is plotted against time. The area under each curve represents the flow volume for a cardiac cycle. In this case, the right and left pulmonary arteries (PA) received 78% and 22% of the pulmonary flow, respectively.

View larger version (19K): [in this window] [in a new window] [Download PPT slide]   Figure 3g.  Differential right and left pulmonary arterial blood flow measurements in a woman with left lung hypoplasia. (a, b) Axial breath-hold gradient-echo cine MR images demonstrate an enlarged right pulmonary artery (R) and a hypoplastic left pulmonary artery (L). To calculate pulmonary arterial flow volumes, VEC MR imaging is performed in planes orthogonal to the right and left pulmonary arteries (black line). (c-g) Blood flow measurements. Magnitude (c, e) and phase (d, f) VEC MR images are obtained through the right (c, d) and left (e, f) pulmonary arteries (arrow). Regions of interest are then drawn around the pulmonary arteries, and flow in these vessels is calculated with the phase images. Ao = aorta. (g) Graph illustrates how arterial flow is plotted against time. The area under each curve represents the flow volume for a cardiac cycle. In this case, the right and left pulmonary arteries (PA) received 78% and 22% of the pulmonary flow, respectively.

View larger version (114K): [in this window] [in a new window] [Download PPT slide]   Figure 4a.  Differential pulmonary arterial flow measurements in a patient with pulmonary artery sling. (a, b) On EKG-gated transverse spin-echo (a) and transverse breath-hold gradient-echo cine (b) MR images, the anomalous left pulmonary artery (arrows) is seen to arise from the right pulmonary artery (R) and course posterior to the trachea (t). Note that the left pulmonary artery is much smaller than the right pulmonary artery. VEC MR imaging is performed in a plane orthogonal to the right and left pulmonary arteries (black line). In this case, a single VEC MR imaging acquisition allowed assessment of both of these arteries because they run parallel for a portion of their course. PA = main pulmonary artery. (c) Magnitude VEC MR image (left) shows the right (R) and left (arrow) pulmonary arteries, around which regions of interest should be drawn. A phase VEC MR image (right) demonstrates low signal intensity in the right pulmonary artery (R) and bright signal intensity in the left pulmonary artery (arrow), indicating that blood in these two vessels flows in opposite directions. (d) Flow profiles of the right and left pulmonary arteries. Graph demonstrates that the biphasic flow profile in the right pulmonary artery is normal, whereas flow in the left pulmonary artery is delayed and diminished, accounting for only 17% of pulmonary blood flow. PA = pulmonary artery.

View larger version (110K): [in this window] [in a new window] [Download PPT slide]   Figure 4b.  Differential pulmonary arterial flow measurements in a patient with pulmonary artery sling. (a, b) On EKG-gated transverse spin-echo (a) and transverse breath-hold gradient-echo cine (b) MR images, the anomalous left pulmonary artery (arrows) is seen to arise from the right pulmonary artery (R) and course posterior to the trachea (t). Note that the left pulmonary artery is much smaller than the right pulmonary artery. VEC MR imaging is performed in a plane orthogonal to the right and left pulmonary arteries (black line). In this case, a single VEC MR imaging acquisition allowed assessment of both of these arteries because they run parallel for a portion of their course. PA = main pulmonary artery. (c) Magnitude VEC MR image (left) shows the right (R) and left (arrow) pulmonary arteries, around which regions of interest should be drawn. A phase VEC MR image (right) demonstrates low signal intensity in the right pulmonary artery (R) and bright signal intensity in the left pulmonary artery (arrow), indicating that blood in these two vessels flows in opposite directions. (d) Flow profiles of the right and left pulmonary arteries. Graph demonstrates that the biphasic flow profile in the right pulmonary artery is normal, whereas flow in the left pulmonary artery is delayed and diminished, accounting for only 17% of pulmonary blood flow. PA = pulmonary artery.

View larger version (91K): [in this window] [in a new window] [Download PPT slide]   Figure 4c.  Differential pulmonary arterial flow measurements in a patient with pulmonary artery sling. (a, b) On EKG-gated transverse spin-echo (a) and transverse breath-hold gradient-echo cine (b) MR images, the anomalous left pulmonary artery (arrows) is seen to arise from the right pulmonary artery (R) and course posterior to the trachea (t). Note that the left pulmonary artery is much smaller than the right pulmonary artery. VEC MR imaging is performed in a plane orthogonal to the right and left pulmonary arteries (black line). In this case, a single VEC MR imaging acquisition allowed assessment of both of these arteries because they run parallel for a portion of their course. PA = main pulmonary artery. (c) Magnitude VEC MR image (left) shows the right (R) and left (arrow) pulmonary arteries, around which regions of interest should be drawn. A phase VEC MR image (right) demonstrates low signal intensity in the right pulmonary artery (R) and bright signal intensity in the left pulmonary artery (arrow), indicating that blood in these two vessels flows in opposite directions. (d) Flow profiles of the right and left pulmonary arteries. Graph demonstrates that the biphasic flow profile in the right pulmonary artery is normal, whereas flow in the left pulmonary artery is delayed and diminished, accounting for only 17% of pulmonary blood flow. PA = pulmonary artery.

View larger version (25K): [in this window] [in a new window] [Download PPT slide]   Figure 4d.  Differential pulmonary arterial flow measurements in a patient with pulmonary artery sling. (a, b) On EKG-gated transverse spin-echo (a) and transverse breath-hold gradient-echo cine (b) MR images, the anomalous left pulmonary artery (arrows) is seen to arise from the right pulmonary artery (R) and course posterior to the trachea (t). Note that the left pulmonary artery is much smaller than the right pulmonary artery. VEC MR imaging is performed in a plane orthogonal to the right and left pulmonary arteries (black line). In this case, a single VEC MR imaging acquisition allowed assessment of both of these arteries because they run parallel for a portion of their course. PA = main pulmonary artery. (c) Magnitude VEC MR image (left) shows the right (R) and left (arrow) pulmonary arteries, around which regions of interest should be drawn. A phase VEC MR image (right) demonstrates low signal intensity in the right pulmonary artery (R) and bright signal intensity in the left pulmonary artery (arrow), indicating that blood in these two vessels flows in opposite directions. (d) Flow profiles of the right and left pulmonary arteries. Graph demonstrates that the biphasic flow profile in the right pulmonary artery is normal, whereas flow in the left pulmonary artery is delayed and diminished, accounting for only 17% of pulmonary blood flow. PA = pulmonary artery.

Quantification of Shunts
VEC MR imaging can also be used to assess the severity of shunt lesions (5,18,19). The technique is used to measure blood flow simultaneously in the ascending aorta and the main pulmonary artery. Pulmonary and systemic flow rates are approximately equal in the absence of a shunt (5,19). If a patient has an atrial septal defect, partial anomalous pulmonary venous connection, or ventricular septal defect with a left-to-right shunt, flow in the pulmonary artery will be greater than flow in the aorta by an amount equal to shunt volume (Fig 5). These measurements can also be expressed as the ratio between pulmonary and systemic flow (Q_p/Q_s). With a patent ductus arteriosus and a left-to-right shunt, aortic blood flow is greater than pulmonary flow, and shunt volume is calculated by subtracting pulmonary flow from aortic flow. The opposite flow relationships hold true for right-to-left shunts.

View larger version (121K): [in this window] [in a new window] [Download PPT slide]   Figure 5a.  Shunt quantification. (a) EKG-gated transverse spin-echo MR image demonstrates a supracristal ventricular septal defect (arrowhead). (b, c) Sagittal EKG-gated spin-echo MR images are used to prescribe the VEC MR imaging sequences that are performed in planes (white line) orthogonal to the main pulmonary artery (PA) (b) and the ascending aorta (A) (c). (d, e) On magnitude (left) and phase (right) VEC MR images, regions of interest are drawn around the main pulmonary artery (PA) (d) and aorta (A) (e) to derive flow volume in these vessels. (f) Graph illustrates the flow profiles of the main pulmonary artery (PA) and aorta. The volume of the left-to-right shunt is calculated by subtracting aortic flow from pulmonary flow. In this case, the ratio between pulmonary and systemic flow (Qp/Qs) was 1.7. The shunt was of borderline severity, and the patient elected not to undergo surgery.

View larger version (173K): [in this window] [in a new window] [Download PPT slide]   Figure 5b.  Shunt quantification. (a) EKG-gated transverse spin-echo MR image demonstrates a supracristal ventricular septal defect (arrowhead). (b, c) Sagittal EKG-gated spin-echo MR images are used to prescribe the VEC MR imaging sequences that are performed in planes (white line) orthogonal to the main pulmonary artery (PA) (b) and the ascending aorta (A) (c). (d, e) On magnitude (left) and phase (right) VEC MR images, regions of interest are drawn around the main pulmonary artery (PA) (d) and aorta (A) (e) to derive flow volume in these vessels. (f) Graph illustrates the flow profiles of the main pulmonary artery (PA) and aorta. The volume of the left-to-right shunt is calculated by subtracting aortic flow from pulmonary flow. In this case, the ratio between pulmonary and systemic flow (Qp/Qs) was 1.7. The shunt was of borderline severity, and the patient elected not to undergo surgery.

View larger version (173K): [in this window] [in a new window] [Download PPT slide]   Figure 5c.  Shunt quantification. (a) EKG-gated transverse spin-echo MR image demonstrates a supracristal ventricular septal defect (arrowhead). (b, c) Sagittal EKG-gated spin-echo MR images are used to prescribe the VEC MR imaging sequences that are performed in planes (white line) orthogonal to the main pulmonary artery (PA) (b) and the ascending aorta (A) (c). (d, e) On magnitude (left) and phase (right) VEC MR images, regions of interest are drawn around the main pulmonary artery (PA) (d) and aorta (A) (e) to derive flow volume in these vessels. (f) Graph illustrates the flow profiles of the main pulmonary artery (PA) and aorta. The volume of the left-to-right shunt is calculated by subtracting aortic flow from pulmonary flow. In this case, the ratio between pulmonary and systemic flow (Qp/Qs) was 1.7. The shunt was of borderline severity, and the patient elected not to undergo surgery.

View larger version (144K): [in this window] [in a new window] [Download PPT slide]   Figure 5d.  Shunt quantification. (a) EKG-gated transverse spin-echo MR image demonstrates a supracristal ventricular septal defect (arrowhead). (b, c) Sagittal EKG-gated spin-echo MR images are used to prescribe the VEC MR imaging sequences that are performed in planes (white line) orthogonal to the main pulmonary artery (PA) (b) and the ascending aorta (A) (c). (d, e) On magnitude (left) and phase (right) VEC MR images, regions of interest are drawn around the main pulmonary artery (PA) (d) and aorta (A) (e) to derive flow volume in these vessels. (f) Graph illustrates the flow profiles of the main pulmonary artery (PA) and aorta. The volume of the left-to-right shunt is calculated by subtracting aortic flow from pulmonary flow. In this case, the ratio between pulmonary and systemic flow (Qp/Qs) was 1.7. The shunt was of borderline severity, and the patient elected not to undergo surgery.

View larger version (143K): [in this window] [in a new window] [Download PPT slide]   Figure 5e.  Shunt quantification. (a) EKG-gated transverse spin-echo MR image demonstrates a supracristal ventricular septal defect (arrowhead). (b, c) Sagittal EKG-gated spin-echo MR images are used to prescribe the VEC MR imaging sequences that are performed in planes (white line) orthogonal to the main pulmonary artery (PA) (b) and the ascending aorta (A) (c). (d, e) On magnitude (left) and phase (right) VEC MR images, regions of interest are drawn around the main pulmonary artery (PA) (d) and aorta (A) (e) to derive flow volume in these vessels. (f) Graph illustrates the flow profiles of the main pulmonary artery (PA) and aorta. The volume of the left-to-right shunt is calculated by subtracting aortic flow from pulmonary flow. In this case, the ratio between pulmonary and systemic flow (Qp/Qs) was 1.7. The shunt was of borderline severity, and the patient elected not to undergo surgery.

View larger version (27K): [in this window] [in a new window] [Download PPT slide]   Figure 5f.  Shunt quantification. (a) EKG-gated transverse spin-echo MR image demonstrates a supracristal ventricular septal defect (arrowhead). (b, c) Sagittal EKG-gated spin-echo MR images are used to prescribe the VEC MR imaging sequences that are performed in planes (white line) orthogonal to the main pulmonary artery (PA) (b) and the ascending aorta (A) (c). (d, e) On magnitude (left) and phase (right) VEC MR images, regions of interest are drawn around the main pulmonary artery (PA) (d) and aorta (A) (e) to derive flow volume in these vessels. (f) Graph illustrates the flow profiles of the main pulmonary artery (PA) and aorta. The volume of the left-to-right shunt is calculated by subtracting aortic flow from pulmonary flow. In this case, the ratio between pulmonary and systemic flow (Qp/Qs) was 1.7. The shunt was of borderline severity, and the patient elected not to undergo surgery.

Because the VEC MR imaging technique relies on calculation of the stroke volume into the aorta and main pulmonary artery, coexisting aortic or pulmonary insufficiency can result in miscalculation of shunt volume unless the estimated stroke volume is reduced by the volume of the regurgitation.

Evaluation of Valvular Disease
Aortic or pulmonary regurgitation can be quantified directly with VEC MR imaging to estimate the volume of antegrade flow during systole and of regurgitant flow during diastole (21,22). Flow is measured in the main pulmonary artery to assess pulmonary regurgitation (Fig 6) and in the proximal aorta to assess aortic regurgitation. Mitral regurgitation can be estimated by subtracting the flow into the aorta during systole from the flow across the mitral valve into the left ventricle during diastole (23).

View larger version (177K): [in this window] [in a new window] [Download PPT slide]   Figure 6a.  Assessment of pulmonary regurgitation in a patient who had undergone right ventricular outflow tract surgery for repair of tetralogy of Fallot. (a) A sagittal EKG-gated spin-echo MR image was used to prescribe the VEC MR imaging sequence, which was performed in a plane (white line) perpendicular to the main pulmonary artery (PA). RV = right ventricle. (b) Phase VEC MR image obtained through the main pulmonary artery (PA) during systole reveals low signal intensity within the pulmonary artery, a finding that indicates antegrade flow. (c) Phase VEC MR image obtained through the main pulmonary artery (PA) during diastole shows bright signal intensity in the pulmonary artery, a finding that indicates retrograde flow. (d) Graph illustrates the flow profile in the main pulmonary artery. Antegrade flow is indicated by a positive flow rate and retrograde flow by a negative flow rate. The regurgitant fraction is computed with the formula RF = FR/FA, where RF = regurgitant fraction, FR = retrograde flow, and FA = antegrade flow. In this case, the regurgitant fraction was 42%, a finding that suggested the need for pulmonary valve replacement surgery.

View larger version (137K): [in this window] [in a new window] [Download PPT slide]   Figure 6b.  Assessment of pulmonary regurgitation in a patient who had undergone right ventricular outflow tract surgery for repair of tetralogy of Fallot. (a) A sagittal EKG-gated spin-echo MR image was used to prescribe the VEC MR imaging sequence, which was performed in a plane (white line) perpendicular to the main pulmonary artery (PA). RV = right ventricle. (b) Phase VEC MR image obtained through the main pulmonary artery (PA) during systole reveals low signal intensity within the pulmonary artery, a finding that indicates antegrade flow. (c) Phase VEC MR image obtained through the main pulmonary artery (PA) during diastole shows bright signal intensity in the pulmonary artery, a finding that indicates retrograde flow. (d) Graph illustrates the flow profile in the main pulmonary artery. Antegrade flow is indicated by a positive flow rate and retrograde flow by a negative flow rate. The regurgitant fraction is computed with the formula RF = FR/FA, where RF = regurgitant fraction, FR = retrograde flow, and FA = antegrade flow. In this case, the regurgitant fraction was 42%, a finding that suggested the need for pulmonary valve replacement surgery.

View larger version (125K): [in this window] [in a new window] [Download PPT slide]   Figure 6c.  Assessment of pulmonary regurgitation in a patient who had undergone right ventricular outflow tract surgery for repair of tetralogy of Fallot. (a) A sagittal EKG-gated spin-echo MR image was used to prescribe the VEC MR imaging sequence, which was performed in a plane (white line) perpendicular to the main pulmonary artery (PA). RV = right ventricle. (b) Phase VEC MR image obtained through the main pulmonary artery (PA) during systole reveals low signal intensity within the pulmonary artery, a finding that indicates antegrade flow. (c) Phase VEC MR image obtained through the main pulmonary artery (PA) during diastole shows bright signal intensity in the pulmonary artery, a finding that indicates retrograde flow. (d) Graph illustrates the flow profile in the main pulmonary artery. Antegrade flow is indicated by a positive flow rate and retrograde flow by a negative flow rate. The regurgitant fraction is computed with the formula RF = FR/FA, where RF = regurgitant fraction, FR = retrograde flow, and FA = antegrade flow. In this case, the regurgitant fraction was 42%, a finding that suggested the need for pulmonary valve replacement surgery.

View larger version (22K): [in this window] [in a new window] [Download PPT slide]   Figure 6d.  Assessment of pulmonary regurgitation in a patient who had undergone right ventricular outflow tract surgery for repair of tetralogy of Fallot. (a) A sagittal EKG-gated spin-echo MR image was used to prescribe the VEC MR imaging sequence, which was performed in a plane (white line) perpendicular to the main pulmonary artery (PA). RV = right ventricle. (b) Phase VEC MR image obtained through the main pulmonary artery (PA) during systole reveals low signal intensity within the pulmonary artery, a finding that indicates antegrade flow. (c) Phase VEC MR image obtained through the main pulmonary artery (PA) during diastole shows bright signal intensity in the pulmonary artery, a finding that indicates retrograde flow. (d) Graph illustrates the flow profile in the main pulmonary artery. Antegrade flow is indicated by a positive flow rate and retrograde flow by a negative flow rate. The regurgitant fraction is computed with the formula RF = FR/FA, where RF = regurgitant fraction, FR = retrograde flow, and FA = antegrade flow. In this case, the regurgitant fraction was 42%, a finding that suggested the need for pulmonary valve replacement surgery.